It is widely known that calcium sulfate hemihydrate occurs in two forms, alpha type calcium sulfate hemihydrate and beta type calcium sulfate hemihydrate. The alpha type calcium sulfate hemihydrate is such that the plasticization thereof which is effected by addition of water can be obtained with a small amount of water. The plasticized alpha type calcium sulfate hemihydrate has a property such that, when molded and left to cure, it produces a shaped article of high strength. It is also known that the normal consistency of the alpha type calcium sulfate hemihydrate having a large crystal size and a high bulk density is small and accordingly the shaped article produced from such calcium sulfate hemihydrate has higher strength. In contrast, the beta type calcium sulfate hemihydrate has high normal consistency and the shaped article produced from this calcium sulfate hemihydrate has low strength.
As examples of the methods heretofore employed for the commercial production of alpha type calcium sulfate hemihydrate, there may be cited a pressurized steam method which includes exposing a naturally occurring gypsum or a chemically produced gypsum to hot steam under increased pressure so as to dehydrate the gypsum dihydrate, a pressurized aqueous solution method which includes converting a naturally occurring gypsum or a chemically produced gypsum into slurry by addition of water or an aqueous solution of crystallization accelerator and heating the slurry under increased pressure while keeping the slurry in an agitated state, and a method which combines these two methods. These methods, however, cannot be called advantageous from the commercial point of view, gfor they invariably are operated under pressure and consequently necessitate use of pressurized reaction vessels.
The situation has encouraged proposal of various methods directed to the production of alpha type calcium sulfate hemihydrate under atmospheric pressure (namely, in the open air) in order to replace the above-mentioned conventional methods which involve treatments under increased pressure. For example, there has been suggested a method which effects the production of alpha type calcium sulfate hemihydrate by suspending gypsum in an aqueous solution containing a soluble inorganic salt such as magnesium sulfate, magnesium chloride, sodium chloride or calcium chloride, an inorganic acid such as sulfuric acid, nitric acid or phosphoric acid or an alkali metal salt of an organic acid and heating the resultant suspension at a temperature falling in the range from 80.degree. C to the boiling point of the aqueous solution. This method allows of a modified embodiment which additionally includes incorporating into the aqueous solution a small amount of an organic acid such as citric acid, tartaric acid or palmitic acid or a salt or ester thereof or an organosulfonic acid or a salt or ester thereof to serve as a crystallization accelerator. However, these atmospheric pressure methods have not yet been perfected to the extent of being commercialized, because the shaped articles molded of the alpha type calcium sulfate hemihydrate produced by such methods are still deficient in strength.
In the atmospheric pressure method described above, use of the aqueous solution containing an alkali metal salt of organic acid as a medium for the dehydration reaction of gypsum is especially recommended for the purpose of lowering the dehydration reaction temperature. Either the sodium salt or potassium salt of an organic acid is preferably used as the alkali metal salt of organic acid. The alpha type calcium sulfate hemihydrate which is produced in this case exhibits extremely inferior physical properties, because the individual crystals making up the gypsum are small and the bulk density of the calcium sulfate is low and the gypsum itself contains the alkali metal salt such as in the form of double-salt eutectic crystals (for example, gypsum + sodium sulfate).
An exhaust gas such as the combustion exhaust gas discharged from a thermal power station contains sulfur oxides. When this exhaust gas is released untreated into the atmosphere, it may possibly cause air pollution. Thus, the removal of sulfur oxides from such exhaust gas has become an established custom in recent years. The sulfur oxides thus removed from the exhaust gas are mostly recovered in the form of gypsum. The gypsum thus recovered from the exhaust gas amounts to colossal volumes. In these circumstances, therefore, the development of new uses for the gypsum forms an important task. As one such use, the alpha type calcium sulfate hemihydrate has received keen attention. Since the conventional methods which involve treatments under pressure are not suitable for the treatment of huge volumes of gypsum, perfection of an advantageous atmospheric pressure method which is suited for the treatment of such gypsum has been a great desideratum.